Known infrared emitters comprise, inside the cladding tube made of glass, a coil-shaped resistor wire or a resistor tape as a heating conductor or heating filament. The wire or the tape has no or essentially no contact to the cladding tube. The heat transfer from the resistor wire to the cladding tube takes place essentially by thermal radiation. The heating conductor, also called a heating filament, is used as a current-conducting incandescent filament, glow wire or glow coil in incandescent lamps, in infrared emitters or in furnaces, and is usually present in an elongated form as a tape that is flat or twisted about its longitudinal axis or is coiled. Carbon fiber-based heating elements show good mechanical stability along with relatively high electrical resistance, and they allow for comparably rapid temperature changes.
In infrared emitters of this type, an electrical resistor element made of a resistor material is the actual infrared-emitting element of the emitter. The cladding tube made of quartz glass is essentially pervious to infrared radiation such that the radiation emitted by the resistor element is transferred to the heating goods without major loss of radiation.
Regarding the electrical properties, a special focus is on the electrical resistance of the heating filament. On the one hand, the electrical resistance should be constant over time even during exposure to load and, on the other hand, it should be as high as possible to be able to operate even short lengths of heating filament with common voltages (for example 230 V).
In the case of a tape-shaped heating filament, the nominal electrical resistance can be adjusted, as a matter of rule, by the cross-section and, in particular, by the thickness of the tape. However, the thickness of the tape can be reduced only to a limited extent considering the mechanical stability and a given minimum service life. This limitation is noticeable especially if the heating filament in-use is exposed to high mechanical loads such as if the irradiation lengths are 1 m or more.
An infrared emitter with a tape-shaped carbon heating filament is known, for example, from DE 100 29 437 A1. The coiled carbon tape is situated at a distance from the wall of the cladding tube and is arranged along the central axis thereof. Contacts with connecting lugs are provided on the ends of the carbon tape and are guided through a crimping area of the cladding tube to the external electrical connectors. The inside of the cladding tube is evacuated during installation in order to prevent changes to the resistance of the heating element due to oxidation. The power density of the carbon emitter is relatively high due to the large surface area of the coiled carbon tape as compared to infrared emitters comprising metallic heating elements. Accordingly, they are also suitable, in principle, for applications in which the emitter lengths are limited to less than one meter. However, it is a problem that the coiled tape causes the emission characteristics to not be fully homogeneous, but to comprise areas of higher power density (so-called hotspots) and of lower power density (cold spots). This problem must be taken into consideration during their use, in particular, for panel radiators by making the emission more homogeneous by keeping a larger distance from the heating goods. However, this measure is at the expense of the efficiency of the emitter.
Besides the infrared emitters with a carbon heating filament, emitters with so-called Kanthal® heating elements are known. They show a broadband infrared spectrum and are typically operated at temperatures of up to 1,000° C. The disadvantages in terms of the emission characteristics lacking homogeneity are similar to what has been described above for emitters with a carbon heating filament.
An infrared heater with a Kanthal coil is known, for example, from U.S. Pat. No. 3,699,309. The Kanthal coil is situated in a cladding tube made of glass and is supported on a cylindrical rod that has a semi-circular cross-section and is made of a ceramic fiber material (Al2O3—SiO2). This kind of support is to prevent “hot spots” of the Kanthal coil. This support is disadvantageous in that the emission range of the infrared emitter is no longer 360° radially according to the circumference of the cladding tube, but rather is reduced by the area of the support rod that contacts the Kanthal coil.